Many radio communications systems, such as used with cellular telephones, use a central base station antenna. Such a base station antenna must have an omnidirectional antenna pattern for transmission and reception in all directions. It is further desirable that antennas of this type have a narrow beam directed laterally toward the users rather than being directed upward and thereby wasted.
Prior omnidirectional antennas are shown in U.S. Pat. No. 4,369,449 to MacDougall; U.S. Pat. No. 4,117,490 to Arnold et al.; and U.S. Pat. No. 3,159,838 to Facchine. The patent to MacDougall describes a linearally polarized omnidirectional antenna. This antenna has one or more dipoles each having an elongated tubular conductive radiator of a length that is about half the wave length of the mid-band frequency. The antenna includes a mast or center tube which is electrically isolated from the cylindrical radiator along the entire length of the radiator. The antenna feed structure is positioned totally within mast with connection points to the radiators at the termination of the feed line. The patent to Arnold et al. describes an antenna array wherein an antenna structure includes spaced concentric cylindrical metal sleeves comprising an outer sleeve and an inner sleeve of equal length. The array comprises two of the antenna structures, one mounted on each strut of the landing gear of an aircraft. The patent to Facchine describes a vertically stacked dipole radiator which is mounted on a tubular mast. This antenna includes a plurality of dipole radiators. The dipole radiators are conical structures which have facing back-to-back closed ends.
The present invention is an improved antenna over the prior art. The antenna of the present invention provides an improved antenna pattern, less complexity, reduced cost of manufacture, greater lightning protection and improved repairability.